It is designed especially for the brake circuits of motor vehicles equipped with a wheel anti-lock system. Known anti-lock systems make use, between a source of fluid under pressure, such as an electric pump or a so-called "full-power" master cylinder, and a pressure receiver, such as a brake motor, of a solenoid valve controlled by a computer as a function of different signals representing particularly the rotational speed of the wheels of the vehicle, in order to relieve the fluid pressure in the brake motor when the computer detects the imminent locking of a wheel and then to increase this pressure again when the computer detects too high a rotational speed of the wheel, until the wheel once again shows a tendency to lock, the intake/relief cycles then being repeated.
The current trend is to take advantage of this wheel anti-lock equipment to make it operate also as a system for preventing the slip of the driving wheels of the vehicle. The computer then commands an increase of pressure in a brake motor of a driving wheel, when it detects a rotational speed of this wheel which is too high in relation to that of a non-driving wheel, and a relief of the pressure in this brake motor, when the speed of the driving wheel is in the neighborhood of that of a non-driving wheel.
Such systems employ two-position solenoid valves, that is to say functioning in the all-or-nothing mode, which give rise to numerous disadvantages for the anti-lock/anti-slip system. In fact, during the intake/relief cycles, the solenoid valve changes from one of its positions to the other many times, and this gives rise to considerable noise generated by the movable assembly of the solenoid valve. On the other hand, pressure peaks occur in the brake motors, thus bringing about transient states which cannot be controlled by the computer. Moreover, these systems are generally highly complex and require numerous solenoid valves, thereby aggravating the disadvantages mentioned above.
The object of the present invention is, therefore, to remedy these disadvantages by providing a pressure-regulating device for a hydraulic circuit, for example for motor-vehicle brake circuits, which allows a wheel anti-lock and anti-slip operating mode and is simple and reliable and the number of solenoid valves of which is reduced to a minimum, in order, furthermore, to reduce the cost of such systems.
To achieve this, the invention provides for the use of a so-called proportional solenoid valve ensuring in the user circuit a hydraulic pressure which is a function of the current circulating in the coil of the solenoid valve within a specific range of movement of the core of the solenoid valve.
Such a solenoid valve then has the advantage that it can be controlled simply by varying the electrical current circulating in the coil, without the need for beating of the movable part; this therefore results in a marked decrease of the pressure peaks in the brake motors. Moreover, such a solenoid valve can easily be controlled by computer ensuring a variable-frequency cutoff of a direct current, such as is found on a motor vehicle, or a fixed-frequency cut-off with a variable cyclic ratio, the intensity of the current integrated by the coil then being a function of the cyclic ratio.
Such a solenoid valve is described for example in U.S. patent application No. 4,744,389. However, the solenoid valve described in this document cannot be arranged in a hydraulic wheel anti-lock brake circuit because of the pressures employed, particularly during the energization of the solenoid valve. Indeed, the solenoid valve of this document is designed for operating permanently in a system for regulating a relatively low pressure. This physical limitation therefore prevents the use of such a solenoid valve with high pressures in the hydraulic motor. Moreover, this solenoid valve does not make it possible to increase the pressure linearly when the current decreases linearly in the coil, and vice versa.
Known anti-slip systems, although using many components of the anti-lock systems, such as the wheel-speed sensors, computer, source of fluid under pressure, etc., nevertheless require additional components, such as other solenoid valves, a specific source of fluid under pressure, etc.
The present invention therefore provides for using for the anti-slip operating mode the proportional solenoid valves which are used for the anti-lock operating mode by means of a differential pressure/vacuum valve.
This thus reduces the number of solenoid valves to a minimum, namely one per wheel, for a combined anti-lock/anti-slip system.